The mechanism - vesicular transport or opened tight junctions - whereby the blood-brain barrier (BBB) is opened in response to hyperosmotic conditions, is still uncertain. Endocytosis by endothelial cells in vitro is halted at 0.5 C. Thus, in order to diminish, physiologically, vesicular transport by cerebral endothelium in situ, hibernating ground squirrels, kept at 8 C, were used. The brain-core temperature in 13 squirrels ranged from 4 C to 12 C. Cold 2.9 ml of 0.8M arabinose solution was infused into one internal carotid artery. One minute later, 35 mg of horseradish peroxidase (HRP) in 0.6 ml of cold balanced salt solution alone or followed by 1 ml of cold 5 mM lanthanum chloride, was infused. The brains were fixed 15 min later with cold aldehydes. In normothermic animals so treated, the BBB is opened: exudates of HRP and ionic lanthanum appear around arterioles and capillaries throughout the brain on the infused side. The experiment was repeated in a normothermic group (brain temp. about 35 C). The consistent result in all groups was HRP exudation on the pial surface. In hypothermic animals, HRP also exuded primarily around large, penetrating vessels but did not spread into the parenchyma. The pial vessels may have exuded HRP which then entered the perivascular spaces around these larger vessels, from the subarachnoid space. Most parenchymal vessels were unaffected. Arabinose and HRP reached the entire vasculature, because in 2 bed after immersion fixation. The results, so far, signify that, since vesicular transport is diminished in hypothermic animals, where the barrier in most vessels in unaffected, barrier opening is due to vesicular transport. However, according to some authors, reduced temperature "stiffens" the cell membrane which cannot invaginate to form vessels. A second interpretation is, then, equally plausible: the tight junctions between cell membranes are less prone to be deformed at 4-12 C during cell shrinkage.